Yes. You can monitor the websites visited by users connected to your WiFi. You can create an OpenDNS account and then change the router server to that of the OpenDNS. This way you can know the websites visited.
Routers like Netgear also have options to view the logs and see each websites visited.
You should know that this is illegal in some countries because it means you are intruding into the person privacy.
Please check and make sure it complies with your local laws before taking any steps.